Strip '/' from api_url string for PXE driver

Because ironic_api_url PXE option used in hardcoded script for posting
callback data we should use only url string without '/' on the end.
It is necessary for case if Ironic API URL option set like
"http://127.0.0.1:6385/".

Change-Id: Iddd237e8aa90fd17f836cfd2060468f34e0de232
Merge "Strip '/' from api_url string for PXE driver"